RE: [PATCH v2 2/2] exfat: check if cluster num is valid

Sungjong Seo
2022-05-15T14:52:15Z

> Syzbot reported slab-out-of-bounds read in exfat_clear_bitmap.
> This was triggered by reproducer calling truncute with size 0, which
> causes the following trace:
>
> BUG: KASAN: slab-out-of-bounds in exfat_clear_bitmap+0x147/0x490
> fs/exfat/balloc.c:174 Read of size 8 at addr ffff888115aa9508 by task syz-
> executor251/365
>
> Call Trace:
> __dump_stack lib/dump_stack.c:77 [inline] dump_stack_lvl+0x1e2/0x24b
> lib/dump_stack.c:118
> print_address_description+0x81/0x3c0 mm/kasan/report.c:233
> __kasan_report mm/kasan/report.c:419 [inline]
> kasan_report+0x1a4/0x1f0 mm/kasan/report.c:436
> __asan_report_load8_noabort+0x14/0x20 mm/kasan/report_generic.c:309
> exfat_clear_bitmap+0x147/0x490 fs/exfat/balloc.c:174
> exfat_free_cluster+0x25a/0x4a0 fs/exfat/fatent.c:181
> __exfat_truncate+0x99e/0xe00 fs/exfat/file.c:217
> exfat_truncate+0x11b/0x4f0 fs/exfat/file.c:243
> exfat_setattr+0xa03/0xd40 fs/exfat/file.c:339
> notify_change+0xb76/0xe10 fs/attr.c:336
> do_truncate+0x1ea/0x2d0 fs/open.c:65
>
> Add checks to validate if cluster number is within valid range in
> exfat_clear_bitmap() and exfat_set_bitmap()
>
> Cc: Namjae Jeon <linkinjeon@kernel.org>
> Cc: Sungjong Seo <sj1557.seo@samsung.com>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: stable@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
>
> Link: https://protect2.fireeye.com/v1/url?k=24a746d8-45dcec51-24a6cd97-
> 74fe48600034-8e4653a49a463f3c&q=1&e=0efc824d-6463-4253-9cd7-
> ce3199dbf513&u=https%3A%2F%2Fsyzkaller.appspot.com%2Fbug%3Fid%3D50381fc738
> 21ecae743b8cf24b4c9a04776f767c
> Reported-by: syzbot+a4087e40b9c13aad7892@syzkaller.appspotmail.com
> Fixes: 1e49a94cf707 ("exfat: add bitmap operations")
> Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>

Looks good.
And it seems that WARN_ON() is no longer needed.

Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>

> ---
> v2:
> - Use is_valid_cluster() helper to validate clu
> ---
> fs/exfat/balloc.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/fs/exfat/balloc.c b/fs/exfat/balloc.c index
> 03f142307174..92f5b5b5a0d0 100644
> --- a/fs/exfat/balloc.c
> +++ b/fs/exfat/balloc.c
> @@ -149,6 +149,9 @@ int exfat_set_bitmap(struct inode *inode, unsigned int
> clu, bool sync)
> struct exfat_sb_info *sbi = EXFAT_SB(sb);
>
> WARN_ON(clu < EXFAT_FIRST_CLUSTER);
> + if (!is_valid_cluster(sbi, clu))
> + return -EINVAL;
> +
> ent_idx = CLUSTER_TO_BITMAP_ENT(clu);
> i = BITMAP_OFFSET_SECTOR_INDEX(sb, ent_idx);
> b = BITMAP_OFFSET_BIT_IN_SECTOR(sb, ent_idx); @@ -167,6 +170,9 @@
> void exfat_clear_bitmap(struct inode *inode, unsigned int clu, bool sync)
> struct exfat_mount_options *opts = &sbi->options;
>
> WARN_ON(clu < EXFAT_FIRST_CLUSTER);
> + if (!is_valid_cluster(sbi, clu))
> + return;
> +
> ent_idx = CLUSTER_TO_BITMAP_ENT(clu);
> i = BITMAP_OFFSET_SECTOR_INDEX(sb, ent_idx);
> b = BITMAP_OFFSET_BIT_IN_SECTOR(sb, ent_idx);
> --
> 2.36.1